1. Field of the Invention
The present invention relates to a backlight unit for housing a lamp that is a light source of a display device with a backlight, and specifically relates to a backlight unit preferably used in a liquid crystal display device including a translucent liquid crystal display panel.
2. Description of the Related Art
A liquid crystal display device and the like including a translucent liquid crystal display panel, which are cited as an example of a flat-screen display device, generally have a backlight unit arranged behind the liquid crystal display panel. The backlight unit is a device including a tubular lamp such as a cold cathode tube as a light source, which controls the properties of light emitted from the tubular lamp and projects the light toward a rear side of the display panel. The projected light passes through the display panel, making an image displayed visible on a front side of the display panel.
FIG. 4 is an exploded perspective view schematically illustrating relevant parts of a structure of a generally conventional liquid crystal display device. A liquid crystal display device 30 includes a bezel 31, a display panel 32 and a backlight unit 33. The bezel 31 is a member that defines a frame of the display panel 32, and the display panel 32 is made by bonding two panels of glass so as to seal in a liquid crystal therebetween.
The backlight unit 33 includes a frame 34, optical sheets 35, a diffusion plate 36, tubular lamps 37, a reflector 38 and a backlight chassis 39. The frame 34 is shaped like a picture frame and secures the optical sheets 35 and the diffusion plate 36 to the backlight chassis 39. The optical sheets 35 and the diffusion plate 36 are provided for controlling the properties of light which is emitted from the tubular lamps 37 and enters the display panel 32. In this case, the optical sheets 35 are made up of three sheets including a diffusion sheet 35b, a lens sheet 35c and a polarizing reflection sheet 35d, each of which has a thickness of 0.1 mm to 0.3 mm. The diffusion plate 36 has a thickness of about 2 mm.
In this case, right-side and left-side ends of the tubular lamps 37 are inserted into electrode part holders 42 so as to be secured to the backlight chassis 39, as shown in a front view of FIG. 5. The reflector 38 is laid under the tubular lamps 37, for reflecting the light emitted from the tubular lamps 37 toward the display panel 32. In order to improve reflectivity, projections 38a having a crest shape are provided on the reflector 38 so as to be located respectively between tube sections 37a of the tubular lamps 37.
The backlight chassis 39, substantially in the shape of a box, has a lamp housing portion 39a including a bottom portion 39b and side-wall portions 39c and 39d, and support surfaces 39e and 39f extending outward respectively from upper edges of the side-wall portions 39c and 39d. In the backlight chassis 39, the bottom portion 39b and the longer side-wall portions 39c are formed of a member 40 which is prepared by subjecting a metal plate material to plate metal processing, and the shorter side-wall portions 39d are formed of a member 41 which is molded of resin.
The tubular lamp 37 is secured to the lamp housing portion 39a of the backlight chassis 39 with the use of the above-mentioned electrode part holder 42, and also with the use of a member 45 which includes lamp clips 43 for holding the tube section 37a at an approximate midpoint thereof, and a holding pin 44 for preventing the diffusion plate 36 which is arranged above from bending downward to preclude luminance irregularity, as illustrated.
The above-mentioned frame 34 is secured to the support surfaces 39e and 39f of the backlight chassis 39 while interposing the optical sheets 35 and the diffusion plate 36 therebetween. In this case, as shown in FIG. 5, protrusion portions 39g having a square shape are formed at center portions of the support surfaces 39e and 39f, and concave portions 35a and concave portions 36a are formed as a notch at center positions on the edges of the optical sheets 35 and the diffusion plate 36. By fitting the protrusion portions 39g of the support surfaces 39e and 39f into the concave portions 35a of the optical sheets 35 and the concave portions 36a of the diffusion plate 36, the optical sheets 35 and the diffusion plate 36 are positioned and supported on the support surfaces 39e and 39f. Thereby, the diffusion plate 36 having the thickness of 2 mm, which is relatively heavy, is prevented from rattling inside the backlight unit 33.
Incidentally, as a prior art literature relating to the present invention, Japanese Patent Application Unexamined Publication No. 2002-196312 is cited.
In the above-described backlight unit 33, a thermal factor of the tubular lamp 37 that is the light source contributes to thermal expansion or thermal contraction of structural components thereof. As illustrated, the diffusion plate 36 is arranged in close proximity to the tubular lamps 37; therefore, when a certain period of time passes after lightening the tubular lamps 37, for example, the diffusion plate 36 tends to be warped toward the display panel 32 and lose flatness.
Since the liquid crystal display device 30 is generally used in an upright position, the thin optical sheets 35 of which each sheet has a thickness of 0.1 mm to 0.3 mm, suffer a plurality of deformed portions 46 at the time of the thermal expansion, because the concave portions 35a get onto or hitches on the protrusion portions 39g for positioning, as shown in FIG. 6. If the optical sheets 35 suffer the deformed portions 46, a display on the display panel 32 has luminance irregularity.